In many energy-storage systems, the volume available for gas storage is restricted and, indeed, can be a limiting factor for a given technology. For example, the acceptance of natural gas as a transportation fuel has been slow mainly due to the lower energy density of that gas as compared with gasoline. The same is true regarding hydrogen and hydrocarbons for the use in spacecraft propulsion systems and fuel cells. The storage of hydrogen is certainly a critical barrier that needs to be overcome before fuel cells can be widely used in transportation applications. Another gas-storage application is the transportation and handling of highly hazardous gases, e.g. those used in the semiconductor industry. Examples of such gases are phosphine (PH3), boron trifluoride (BF3), and arsine (AsH3). Thus, there exists a strong need for the development of lightweight, compact, safe, and high energy density storage of gases for a variety of applications.
At present, there are two principal approaches to this problem: (a) physical storage (compressed gas, liquefaction, and adsorption); and (b) chemical storage (e.g., metal hydrides for hydrogen storage). The above state-of the art solutions have the following limitations. Compressed gas systems have high weight-to-volume ratios due to the heavy containers required for the storage of pressurized gas. Lightweight graphite composites show some promise in alleviating this difficulty but further advancements and product development are required to assure safe and reliable use of these materials. In addition, the transportation and storage of toxic gases in pressurized containers poses safety concerns associated with accidental tank rupture. Cryogenic storage carries with it significant penalties because of the energy required to liquefy the gas and to maintain it in a liquefied state. Metal hydrides, or other suitable chemicals, remain a possible option, but low-cost hydrides currently require high temperatures to liberate the stored hydrogen. The hydrides which are capable of liberating hydrogen at low temperatures have a very low storage capacity.
Gas storage based on adsorption on activated carbon offers large storage capacity in terms of weight (2-4 times greater than state-of-the-art hydrides). Activated and other carbons with significantly developed nanostructure have a great potential to provide substantially larger storage capacities than gas compression because the density of the adsorbed phase is greater than gas density. The high density of gas at the surface more than compensates some loss of the available volume associated with the introduction of a sorbent. In some cases, the subatmospheric storage of gas adsorbed on microporous sorbent can be more effective, and much safer, than the storage of the same gas in pressurized tanks. This is thanks to the extremely strong interactions between the adsorbed molecules and the walls of micropores (pore size, d<2 nm) present in the adsorbent. The high density of the adsorbed gas is accomplished through the use of monolithic sorbents, e.g. discs, containing mostly micropores, with only small amounts of mesoporosity (pore size, d=2-50 nm) and macroporosity (pore size, d>50 nm).
A key to successful gas-storage sorbents is the ability to “engineer” the pore structure in such a way so that only pores having optimum dimensions are formed. The optimum dimension will, generally speaking, be different for different gases, but the common features of good gas-storage sorbents are: (1) a large number of pores with a narrow pore-size distribution within the micropore region (pores smaller than 2 nm); (2) the absence of meso and macroporosity (pores larger that 2 nm); (3) the absence of voidage within the storage container, i.e., the sorbent has the form of monolithic elements, e.g., discs, rather than loosely packed particles; and (4) the absence of surface complexes, e.g., stable surface oxides, that may block active sites or impede gas diffusion into and out of the pore structure.
Innovative char-activation and pore-size control techniques have heretofore been developed to tailor the pore structure of carbon-based sorbents, as disclosed for example in Quinn and Holland U.S. Pat. No. 5,071,820, 1991, and Wójtowicz, Serio, and Suuberg, U.S. Pat. No. 6,626,981. Further advances in the art are needed however to optimize sorbent properties and performance.